A thin metal plate bonding method generally includes welding, soldering and diffusion bonding processes according to types of the thin metal plates to be bonded.
The welding process includes partially melting a bonding part of metal, then solidifying the same to complete the bonding, while the soldering process uses a bonding material having a lower melting point than a thin metal plate to be bonded, wherein the molten bonding material is evenly infiltrated into a gap between the thin metal plates to be bonded through a capillary phenomenon to bond the same. These two bonding processes are very simple and commercially used in a wide range of applications. However, such techniques could not be used for entirely bonding the interface of a plate to be bonded, hence not assuring air tightness between channels formed inside the plate. Therefore, the above processes cannot be employed as a bonding method for a micro-channel reactor (hereinafter referred to as an MCR), which is configured by thin plate assembly technique.
Meanwhile, the diffusion bonding is a bonding method that varies some parameters such as temperature, pressure, time, etc. in order to generate diffusion between different materials and bonds the same. When using this method, it is advantageously possible to bond not only the thin metal plate but also a thin plate of a non-metal material. When bonding thin plates with being stacked up such as a heat exchanger or micro-channel heater (Korean Patent Registration No. 10-0719484), the bonding can be completed throughout a contact interface between the thin plates to thus attain air tightness between channels. Therefore, it is possible to form a reactor using the above bonding technique.
However, since the conventional diffusion bonding method is performed at high temperature and high pressure, and in a case in which a thin plate to be bonded has a micro-channel made of the same material as of a catalyst, like an MCR apparatus disclosed in Korean Patent Registration No. 10-0719486, there are problems that the catalyst is damaged due to degradation during bonding, and a shape of the micro-channel was changed. Further, there is another problem that micro-pores are present in the bonded thin plate assembly to cause a decrease in air tightness, for example, inter-connection between channels, leakage to an outside, etc.
In regard to the above-described problems, a technical field of a heat exchanger or reactor will be additionally described, for example. The technique of the heat exchanger or reactor using the thin plate may enable scaling-up by repeatedly laminating the same form of thin plates by turns, therefore, being an applicable field that has a configuration involving excellent mass-productivity and a great possibility of further development. The repeatedly laminated configuration of the heat exchanger may be formed by stacking up thin plates in turns, wherein each of the thin plates has microfine grooves processed on the surface thereof through which a fluid can flow, as shown in FIGS. 1 and 2.
The gist of the above configuration is that, when the number of alternately laminated thin plates is increased to several hundreds, the number of bonded interfaces is also increased to several hundreds. Even if only one of the plates has a pore in any position of A, B, C shown in FIG. 2 to cause a leakage of fluid in arrow directions, two different fluids A and B are mixed with each other, otherwise, a fluid C may be discharged to the outside. As a result, the heat exchanger may loss the function of a heat exchanger. The leakage to the outside C may be supplemented by determining a site of the pore using a gas detector, then welding a part around the pore site, however, if there are pores formed inside the areas A and B, it is not possible to supplement the same by, for example, a locally welding process. Accordingly, suppression of pores is the most difficult problem in the present technical field. A cause of pore generation may be avoided by conducting the bonding at a high temperature. However, in such a configuration that a functional component such as a catalyst or separator is included in the heat exchanger, these components have a low heat-resistant temperature and may be lost by degradation during high-temperature bonding, hence causing a problem. For these reasons, there is still a difficulty in manufacturing products although the above method can be utilized in diverse industrial applications and have some advantages.
In order to overcome the conventional problems described above, in addition to establishment of basic requirements such as a desired bonding strength and minimum change of appearance, diffusion bonding should be executed at low temperature and low pressure to prevent degradation of internal components during bonding, and even when the bonding process is conducted under conditions of low temperature and low pressure, high air tightness must be ensured.